Phenol and its derivatives are highly toxic compounds. They are widely used in the production or manufacture of a large variety of aromatic compounds including explosives, fertilisers, lighting gases, paints, rubber, textiles, pharmaceutical products, perfumes and plastics, such as for example polycarbonates or epoxy resins. Phenol is routinely used as an intermediate in synthetic industrial chemical processes and notably in the petroleum, plastics, dye, leather, paper, soap industries. To a lesser degree, phenol and its derivatives are used in the composition of cosmetics and medicinal products. In some countries, phenol is also used as a mosquito repellent and as an insecticide and weed killer for agriculture. Consequently, phenol and its derivatives are found in the environment mostly via industrial emissions, but also via domestic activity (human and animal metabolism, waste water, household fuel oil combustion). Traces of phenol are also present in motor vehicle exhaust gases or tobacco smoke. Since 2006, the use of phenol as a disinfectant has been prohibited in France. However, phenol derivatives, such as chlorophenols for example, are frequently used as a substitute. As such, phenol and its derivatives are omnipresent, in air and in water, at variable concentrations according to human activity.
Phenol is toxic by inhalation, in contact with skin and if swallowed. It is also liable to induce genetic abnormalities. In the event of contact, it causes skin burns and eye lesions. Repeated or prolonged exposure may induce severe effects for organs, such as: digestive disorders, headaches, salivation, anorexia, vomiting and loss of appetite. It also causes bone marrow damage.
The restrictive occupational exposure limits for air in work premises have been set in France and in the European Union to 2 ppm (7.8 mg/m3) for a weighted average over 8 hours and to 4 ppm (15.6 mg/m3) for short-term exposure of not more than 15 min (as per the toxicology file of the French National Institute for Research and Health-INRS).
At the present time, sensitive, selective and reliable detection of phenol in a gas mixture is performed using an indirect method with gas sampling and subsequent analysis in a laboratory.
In order to detect phenol in solution, it is necessary to use a different method, the reference being the colorimetric method based on 4-aminoantipyrine (AAP), wherein phenol cannot be added last, and the reagents, previously stored separately, must be added at the last minute, which renders the method very complex. Finally, these methods can only be used for detection, and under no circumstances for depollution, if the presence of phenol is established.
There are numerous methods for detecting phenol in aqueous solution or in the air.
The detection methods are essentially broken down into three categories: chromatographic methods, electrochemical methods and colorimetric methods. Further methods also exist and are under development such as for example methods based on mass microvariation (piezoelectric resonators).
Chromatographic methods require costly apparatuses and complex procedures. In addition, they induce a response time when they are used indirectly (field sampling following laboratory analysis).
Electrochemical methods are not readily adaptable for detection in air and frequently require the addition of modified electrodes containing nanoparticles, polymers or enzymes. The costs of such electrodes and the ageing thereof under actual conditions of use may constitute an impediment to the industrialisation thereof.
Chromatographic and electrochemical methods exhibit intrinsic complexity either in the raw material (enzymes, nanoparticles, etc.), in the preparation (materials), in the operating protocol thereof (sampling, extraction, concentration, purification, addition of other reagents, assay), or in the measurement per se (complex, bulky apparatuses). With this complexity, a relatively high cost and a potentially long time to obtain results are thus associated. Furthermore, a certain expertise is required to run the samples and interpret the results, as these methods are far from being simple to use.
Colorimetric methods have the advantage of being simpler to use in terms of preparation, sampling, measurement, and obtaining results. These methods are generally direct (no retrospective laboratory analysis). The response is thus quicker. Furthermore, the apparatus required is inexpensive, and more compact.
In the literature, there are a large number of colorimetric methods for detecting phenol or its derivatives in solution, using reagents in an acid medium, such as p-diazobenzenesulphonic acid (DABS) which gives rise to a yellow colour, the mixture Fe(II)/1,10-phenanthroline which produces pink staining, the mixture formaldehyde-sulphuric acid (FSA) and the purple staining thereof, or iodine bromide which forms an absorbent product in the UV range. Reactions in basic medium have also been described, using for example Folin-Ciocalteu's reagent, which develops blue staining. Further reagents require extraction with an organic solvent of the stained product, such as for example the brown product of Millon's reagent, the pink product of MBTH (3-Methyl-2-benzothiazolinone hydrazone hydrochloride monohydrate) in the presence of an oxidant, or the blue load transfer complexes derived from 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). Some of these reagents are costly, others toxic. Furthermore, they may require an extraction step before the colour analysis. They are not all sufficiently sensitive and selective.
The most commonly used method which provides satisfactory sensitivity and selectivity, probably because it is the simplest, is based on the use of 4-aminoantipyrine (AAP). Moreover, it is the reference method cited by the French National Institute for the Industrial Environment and Risks (INERIS). The method involves the formation of a quinoid complex between 4-aminoantipyrine and phenol in a basic medium (preferentially pH=9 or 10) and in the presence of the oxidant potassium hexacyanoferrate. Staining develops after 5 min for concentrations greater than 1 mg/l i.e. approximately 10 μM (193 ppb) and the quinoid compound has an absorption peak around 510 nm. It should be noted that for concentrations below 1 mg/1, INERIS recommends three successive extractions with chloroform, which renders the operation notably more complex. In the literature, this reaction is described with various AAP compounds and various oxidants, and with various [AAP]:[oxidant] ratios, for example Analytical Chimica Acta, 467, 2002, 105-114, Jour. Chem. Soc. Pak., 27, 2005, 271-274, or Jour. Organ. Chem., 8, 1943, 417-428.
The AAP compounds mentioned in some publications are of interest, notably those making it possible to increase sensitivity by means of a higher molar extinction coefficient. However, they are not easy to acquire as they are mostly synthesised on commission. The majority of methods based on AAP described in the literature enable selective and sensitive detection of the order of a few ppb, but they all use a chloroform extraction step which complicates the protocol and prolongs the duration thereof. If seeking to do away with this extraction step, the INRS guidelines and rare publications with direct detection in aqueous solution indicate a significant drop in sensitivity: the method only works correctly for concentrations greater than 200 ppb (1 mg/1, 10 μM). All these methods use excess oxidant, and the order of use of the reagents is always the same, i.e.: 1) buffer solution, 2) phenol, 3) AAP, 4) oxidant. The waiting time before measuring the absorbance varies from 5 to 30 min.
Attempts to adapt this reference phase in liquid phase to the detection of phenol in gas phase are not convincing. Indeed, only two patents have been filed, and adaptation required a significant complication of the method [Patent documents CN102590198 and CN202362242U]. A plurality of steps are added, notably dissolution in acid medium, distillation, stripping, heating, extraction. Furthermore, in one of the patents, the reagents are added sequentially, which complicates the method further [Patent document CN202362242U].
The disclosure above demonstrates that, at the present time, there is no simple and direct colorimetric method based on the use of AAP or its derivatives, which is selective and sensitive, and which enables the detection of phenol in solution by adding same last. In other words, there is no method where the addition of the sample constitutes the final step.
The methods developed for the detection of phenol in air are much less numerous. By an overwhelming majority, known methods are performed in a plurality of steps. Firstly, sampling of air containing phenol is performed via a tube filled with silica gel or resin (typically XAD7), and this sampling is then followed by desorption with solvents and derivatisation with a silylation agent, then, an assay is performed, by gas chromatography with flame ionisation detection or by liquid chromatography with UV or electrochemical detection.
The only direct method, i.e. with no retrospective laboratory analysis, consists of using GASTEC (No. 60) or DRAEGER (Phenol 1/b, MSA Phenol-1) graduated tubes. The detection principle is based on reacting phenol in an acid medium with (NH4)2Ce(NO3)6 (for GASTEC) and Ce(SO4)2 (for DRAEGER), which produces a change of colour from yellow to brown-green. The stained product spreads through the tube with a rate dependent on the phenol concentration for a gas stream pumped at a flow rate defined by the manufacturer. The concentration reading is performed using the graduations printed on the tube. The most effective is the GASTEC tube, with a detection range of 0.4 to 187 ppm with an error of 10% to 15% indicated by the manufacturer. The advantage of this detection is the simplicity thereof, but the detection reaction is not selective, and it is necessary to apply a correction factor based on the humidity and the temperature. As such, these tubes do not provide the selectivity or the precision required for comparison with an occupational exposure limit value as per INRS. These tubes cannot be used in water, which represents a further limitation with the method.
Therefore, at the present time, there is no direct, selective and sensitive colorimetric method suitable for detecting phenol in gas phase. Furthermore, no universal method for the detection both in a gas mixture such as air or in a solution, for example aqueous, by merely contacting a functional material with the fluid, has been developed to date.
It would thus be desirable to have such a method. The method should enable direct, optical or visual detection feasible in the field with portable equipment, to prevent the transport of samples to be tested to a laboratory equipped to make such a measurement. The method should also be simple. It should be suitable for use equally well for the detection of phenol or a phenol derivative in a gaseous or liquid mixture. The method should be suitable if possible not only for use for detection, but also for selective depollution of phenol and its derivatives.